The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Natural resources such as oil or gas residing in a subterranean formation can be recovered via the formation of wells that penetrate the formation. In particular, a wellbore is typically drilled down to the formation while circulating a drilling fluid (also known as a drilling mud) through the wellbore. During the drilling process, the drill bit generates drill cuttings that consist of small pieces of shale and rock. The drilling fluid carries the drill cuttings in a return flow stream back to the well drilling platform. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the well bore. The drilling fluid is then usually circulated downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Subsequent secondary cementing operations, i.e., cementing operations occurring after the primary cementing operation, may also be performed.
Aqueous drilling fluids are commonly employed for downhole operations. The lost circulation of such aqueous fluids is easier to control than that of oil-based drilling fluids. Further, the use of aqueous drilling fluids avoids the environmental issues associated with the clean-up and disposal of drill cuttings coated in oil. Aqueous drilling fluids usually contain clays such as bentonite or polymers such as polysaccharides or polyacrylamides to increase their viscosities. The polymer-based aqueous fluids are becoming increasingly preferred over the clay-based fluids due to the problems associated with the clay-based fluids. For instance, the downhole equipment may be clogged by clay that is present in the drilling fluid.
Unfortunately, the thermal stability of polymer-based aqueous drilling fluids is often compromised as those fluids pass down the wellbore where they are exposed to increasing temperatures. Temperatures in subterranean formations generally rise about 1° C. per hundred feet of depth. Various polymer-based aqueous fluids have their own characteristic temperatures above which they undergo severe chemical degradation, which leads to an undesirable reduction in fluid viscosity. Therefore, such fluids are typically limited to use in downhole operations above a certain depth to ensure that those fluids do not exceed a maximum temperature. Otherwise, the fluids might be unable to suspend drill cuttings therein as they flow back to the surface, and the cuttings could become deposited in undesired locations in the wellbore. Furthermore, the density of the fluids could drop to a level at which they are incapable of withstanding relatively high fluid pressures downhole.
To avoid the problems associated with viscosity reduction in polymer-based aqueous fluids, formates (i.e., organic salts) are commonly added to the fluids to enhance their thermal stability. However, this technology of using the formates is very expensive. A need therefore exists to develop a way to improve the thermal stability of polymer-based aqueous fluids without the need to use formates.